High-voltage devices need a reliable edge termination at the edge of the die to ensure that the devices can reliably block the high voltage. The edge termination shall relieve the electric field strength between the active area and the kerf or sawing edge and shall prevent any excessive field increase at the kerf edge. Typically, the edge terminations are adapted to shape the electrical field such that the potential lines are diverted towards the surface of the device without any strong bending or crowding of the potential lines in order to prevent avalanche generation in the semiconductor substrate or dielectric breakdown in the passivation layers. Critical topological areas in the edge termination structure are steps and edges where peak field strength of up to several MV/cm might be generated.
Planar edge terminations, which are a common technique to reduce the electric field strength, employs field plates arranged on the top surface of the devices or a varying lateral doping to adapt the electric field strength at the surface of the semiconductor device. The space required for planar edge terminations is high to prevent any local increase of the electrical field strength above the critical value for avalanche breakdown. To keep the electrical potential line curvature sufficiently small, a lateral width of about 200-250 μm is needed for an edge termination zone of a device capable for blocking 600 V. For a 6.5 kV blocking voltage, the required lateral width increases to about 2000 μm.
Another approach uses the so-called mesa edge termination where the electrical field strength relief at least partially occurs within the vertical depth of the device to reduce the required lateral space. Mesa edge termination zones may include trenches or bevelled pn-junctions. Raw techniques such as laser processing, lapping, grinding or sand blasting are needed to produce the desired shape of the edge termination zone which techniques are often unsuitable for wafer mass production.
In view of the above, there is a need for improvement.